Fragmentation

# Papers

## 2020 - Mon Not R Astron Soc - PAHs as sources of small hydrocarbons in PDRs

This paper studies the destruction of PAHs of which the dominant product is acetylene (C2H2) at two PDRs (the Orion Bar with high ultraviolet radiation field and the Horsehead nebula with low one). The gas phase formation of C2H2 is compared with the C2H2 production from the PAH dissociation (higher rates in the Orion Bar at Av < 1 and Av >3.5). The opposite trend is observed in the Horsehead nebula. To estimate the production rate of C2H2, the authors applied a chemical model of C2H2 formation for different PAHs at two PDRs. Only the processes of dissociation (C and H losses) and hydrogenation are included in the model excluding PAH formation and growth. As for the loss channels, only H, H2 and C2H2 are considered.

## 2019 - J. Phys. Chem. A - 1,2,3,4-Tetrahydronaphthalene

• Ion Dissociation Dynamics of 1,2,3,4-Tetrahydronaphthalene: Tetralin as a Test Case For Hydrogenated Polycyclic Aromatic Hydrocarbons
• M. Diedhiou, B.J. West, J. Bouwman, and P.M. Mayer, J. Phys. Chem. A 123, 10885 (2019).
• https://doi.org/10.1021/acs.jpca.9b09511

This paper uses experiments (CID, VUV-Induced Dissociation) and calculations (DFT, RRKM) to investigate the dissociation of one system (Tetralin, C10H12 which is naphthalene+4H).

• Experiments
• The main loss channels from CID are H (18%), CH3, C2H4 (40%), C3H5 (16%).
• VUV-Induced dissociation requires 11~13 eV photon energies and has the major loss of C2H4 (60%). The second most fragment is C3H5 (20%) and the H loss is only 5%.
• Calculations
• All structure optimization and harmonic vibrational frequency calculations were performed using B3LYP/6-31+G(d,p) in Gaussian16.
• Not sure about how RRKM reproduces the relative abundance of possible products as a function of photon energy
• Assumption of a two-well potential energy is needed.
• The energy difference of IS and fragment + C2H4 is 1.40 eV at B3LYP level.
• The energy barrier for ring-opening is 1.29 eV
• The C2H4 desorption energy is 0.16 eV
• H must migrate to specific site before C2H4 formation
• The energy needed for single hydrogen loss from different carbon: sp2 C (in the middle between sp2 C and sp3 C) < sp2 C (in the middle between two sp2 C) < sp3 C
• The C2H4 loss requires less energy than H (1.90 eV), CH3 (2.45 eV), C3H5 (2.72 eV).

## 2019 - J. Phys. Chem. A - C2H2 loss

This paper is a computational study on the C2H2 loss from the ions of naphthalene, anthracene, phenanthrene, tetracene, and pyrene.

## 2016 - ApJ - Fragmentation of pristine and superhydrogenated pyrene cations

• Photo-stability of super-hydrogenated PAHs determined by action spectroscopy experiments
• M. Wolf, H.V. Kiefer, J. Langeland, L.H. Andersen, H. Zettergren, H.T. Schmidt, H. Cederquist, and M.H. Stockett, ApJ 832, 24 (2016).
• https://doi.org/10.3847/0004-637X/832/1/24
• Experimental measurement of the number of photons (energy) needed for breaking carbon backbone: measuring the yield of mass-selected photo-fragment ions as function of laser pulse intensity.
• C$_{16}$H$^+ _ {10}$: 3 photons (~3eV)
• C$_{16}$H$^+ _ {16}$: 2 photons
• C$_{16}$H$^+ _ {26}$: 1 photon

## 2015 - Phys. Rev. A - Role of hydrogenation in pyrene C16H10 fragmentation

• Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation
• M. Gatchell, M.H. Stockett, N. de Ruette, T. Chen, L. Giacomozzi, R.F. Nascimento, M. Wolf, E.K. Anderson, R. Delaunay, V. Vizcaino, P. Rousseau, L. Adoui, B.A. Huber, H.T. Schmidt, H. Zettergren, and H. Cederquist, Phys. Rev. A 92, 050702 (2015).

The MD simulations were performed to simulate collisions between (hydrogenated) pyrene and He using the reactive Tersoff potential.

For Mass spectra for collisions between $C_{16}H_{10+m}^+$ (m = 0, 6, 16) and He at 110 eV center-of-mass energy, the major product is still their intact parent ion.

• For pyrene, the larger the degree of hydrogenation, the larger the measured carbon backbone fragmentation cross section
• The dissociation energy of $C_2H_4$ from $C_{16}H_{16}^+$ is 3.88 eV at B3LYP/6-31G(d) level
• For $C_{16}H_{26}^+$, $CH_3$ loss (1.60 eV) has lower barrier than H loss (2.02 eV)
• Molecular dynamics simulations of vibrationally excited native and hydrogenated pyrene cations that are independent of the excitation method.
• In MD simulations between the collision between (hydrogenated) pyrene and He, the cross section of carbon knockout accounts for less than 20% of the measured total carbon backbone fragmentation cross section. How to get the carbon knockout cross section? This leads to the conclusion that statistical fragmentation of molecules vibrationally heated in the collisions is the main cause for carbon-backbone cleavage in the experiments, not knockout processes.

## 2014 - Phys. Rev. Lett. - Dissociation of hydrogenated coronene C24H12

• Deexcitation Dynamics of Superhydrogenated Polycyclic Aromatic Hydrocarbon Cations after Soft-x-Ray Absorption
• G. Reitsma, L. Boschman, M.J. Deuzeman, O. González-Magaña, S. Hoekstra, S. Cazaux, R. Hoekstra, and T. Schlathölter, Phys. Rev. Lett. 113, 053002 (2014).

The paper is organized as follows:

• general disscussion about hydrogenation of carbon-based materials (hydrogen storage, impacts of hydrogen on electronic properties like band gap opening in graphene)
• astronomical interests in hydrogenated PAHs (isolated system): H2 formation, small PAHs formation from the dissociation of large PAHs under harsh interstellar environments
• brief introduction to this work in both experimental and computational aspects
• experiment setups on how to get mass spectra, how to generate unhydrogenated gas phase coronene cations, how to control degree of hydrogenation
• experimental descriptions about how to count ion yields
• features of dication (two positive charges): how dication is generated and photoabsorption channels
• Figure 2 (black line): mass spectra of (coronene, coronene-1H, coronene-5H, coronene-7H) after 285 eV photoabsorption
• Figure 2 (red line): mass spectra of ions (coronene, coronene-1H, coronene-5H, coronene-7H) without photoabsorption
• DFT calculations about H and 2H loss from coronene-7H ion
• Figure 2 (cyan line): simulation of mass spectra using a cascade model
• The first experimental study of the dissociation of superhydrogenated PAHs

I don't understand something in this paper:

• Why X-ray adsorption is used to excite PAHs, why not multi photons or electron impacts?
• What is Auger decay process?
• Coronene ion is only hydrogenated up to 7H. It can be further hydrogenated until fully hydrogenated. Why not further hydrogenation (>7H) is not included in the paper? Are there any difficulties in controlling hydrogen exposure time during the preparation of hydrogenated coronene?

This paper didn't consider the possibility of CxHy loss and further hydrogenation (> 7H). Only one PAH molecule coronene is not enough to say that the hydrogenation can protect PAHs.

## IRMPD review

• Gas-phase infrared multiple photon dissociation spectroscopy of mass-selected molecular ions
• J. Oomens, B.G. Sartakov, G. Meijer, and G. von Helden, Int. J. Mass Spectrom. 254, 1 (2006).
• https://www.sciencedirect.com/science/article/pii/S1387380606002570

This review provides an overview of the infrared spectroscopy of mass-selected gas-phase molecular ions (mainly polycyclic aromatic hydrocarbons, PAHs).

### IRMPD

At a given lasing frequency, the IRMPD process only occurs when the IR irradiation is matching an IR active mode of the species under investigation. Recording the IRMPD efficiency as a function of IR wavelength results thus in an IR spectrum.

### Energy

IRMPD typically requires the absorption of tens to hundreds of photons. The photon energy at 10 μm wavelength (1000 cm-1) is 0.12398 eV, thus the total energy needed is from 1.24 eV to 12.4 eV. Most aromatic systems have an ionization potential ranging from 7 to 8 eV.

### Time scale

FELIX produces radiation in so-called macropulses, typically 5μs long, which consist of a train of 0.1–10 ps long micropulses. The micropulses are spaced by 1 ns. At room temperature, IVR lifetimes of aromatic molecules are typically much less than 1 ns.

### IR calculation

B3LYP with a double zeta basis set is the recommended method here. It cites other old papers to say that this level of theory is able to give a good description of both geometries and vibrations. Only harmonic approximation of considered here, of which the resulting spectrum is a linear absorption spectrum. DFT calculated harmonic frequencies were uniformly scaled with a factor of 0.96 to account for anharmonicity as well as for experimental redshifting of bands caused by multiple photon excitation. This may differ from the experimentally obtained IRMPD spectrum where the multiple photon excitation mechanism matters. Nonetheless, despite these discrepancies for individual species, the general agreement between the experimental IRMPD spectra and the DFT computed linear absorption spectra is reasonably good.

General IR features of PAHs:

• 700–800 cm−1: C-H out-of-plane bending modes
• 1000-1600 cm−1 (fingerprint region): C-H in-plane bending modes and C-C stretching mode
• ~3000 cm-1: C-H stretching mode (usually not covered in IRMPD of PAHs)
• Cationic PAHs have stronger (a factor of 10) IR intensities compared with neutral species.

### Dynamics

It is possible to describe the dynamical effects of IRMPD qualitatively with the presence of following difficulties:

• The infrared induced fragment ions absorb additional photons to undergo subsequent dissociation processes.
• Not only are the dissociation thresholds for the different channels unknown, a more fundamental difficulty is, that after IR excitation, not all ions will have the same internal energy.

### MISC

• Laser based methods can achieve a higher sensitivity and a better spectral resolution, but narrower spectral range than the traditional gas-phase measurements by by simply placing an absorption cell in the light beam.
• "action spectroscopy" technique: not the direct absorption of the incident light beam is measured, but the response of the molecule to photon absorption. Such a response can be, for example, the emission of photons, fragments or electrons.
• Lasing wavelengths are tunable.

## First paper about IRMPD of cationic PAHs

• Gas-Phase Infrared Photodissociation Spectroscopy of Cationic Polyaromatic Hydrocarbons
• J. Oomens, A.J.A. van Roij, G. Meijer, and G. von Helden, Astrophys. J 542, 404 (2000).
• https://iopscience.iop.org/article/10.1086/309545